The present invention pertains to implantable cardiac stimulators for detection and treatment of ventricular arrhythmias, and corresponding methods therefor, and more particularly to a system and method for reliably detecting reentrant ventricular tachycardias.
Although atrial tachycardia (AT) and atrial fibrillation (AF) are generally hemodynamically tolerated because the atria contribute relatively little to cardiac output, special treatment may be necessitated in cases where the patient is symptomatic or at high risk in events of AT or AF. With ventricular tachycardia (VT), cardiac output may be significantly diminished because the ventricles, the main pumping chambers, are only partially filled between the rapid contractions. Moreover, there is a high risk that the VT may accelerate into ventricular fibrillation (VF), spontaneously or in response to treatment of the VT. In that event, there is an instantaneous cessation of cardiac output as a result of the ineffectual quivering of the ventricles. Unless cardiac output is restored almost immediately, tissue begins to die for lack of oxygenated blood, and death will occur within minutes.
It is a principal object of the present invention to provide improved systems and techniques for detecting ventricular tachycardia, and for distinguishing such arrhythmias from normal high rates, to assure rapid delivery of appropriate therapy.
In the Haluska et al application, an implantable cardiac stimulator is disclosed that integrates the functions of bradycardia and anti-tachycardia pacing-type therapies, and cardioversion and defibrillation shock-type therapies, to provide a coordinated approach to the management and treatment of ventricular arrhythmias, including VT and VF. A significant aspect of that approach is to provide flexible sequencing among the therapies, with appropriate regard to hemodynamic tolerance (or intolerance) of the patient to the detected arrhythmia, and sophisticated detection of arrhythmias together with means for distinguishing those episodes for which treatment is required (such as reentrant tachycardias) from those which are not associated with cardiac or other disease (such as exercise-generated sinus tachycardias). The coordinated approach of that invention further takes into account and deals effectively with the risk of acceleration of a tachycardia, as well as with considerations of longevity of the power source for an implantable device, and of patient acceptance of the device.
According to the invention disclosed in the Haluska et al application, a multiplicity of hierarchical detection algorithms and hierarchical therapeutic modalities are selectively available to the physician and applicable to detect and treat classes of ventricular tachycardia according to their respective positions in the heart rate continuum, and thus according to hemodynamic tolerance or intolerance of the patient to the tachycardia, with backup capabilities of defibrillation and bradycardia pacing for cardiac arrhythmias at the respective higher and lower regions of the rate continuum. Aggressiveness of the therapy is increased with elapsed time and with increasing abnormal heart rate. The physician is provided with complete control over the aggressiveness of the therapy for a particular patient and tachyrhythmia, utilizing a hierarchical approach to treatment. The methodology employed in developing the hierarchy is such that physician control is imparted over a wide variety of possible therapy regimens ranging from the basic to the highly complex, with relatively simple programming of the device.
In an embodiment of that invention, the cardiac stimulator permits selective partitioning of the heart rate continuum into a plurality of contiguous tachycardia classes of progressively higher rate ranges, the lowest and highest of these classes being bounded respectively by regions of the continuum denoting sinus rate and fibrillation. Each of the rate ranges and the latter regions may be arbitrarily designated by the physician, as may be necessary to meet the particular needs of the patient's disorder and the flexibility of the therapy regimens to be prescribed. The stimulator includes a hierarchical detection system for detecting cardiac episodes indicative of arrhythmia and for distinguishing between normal and abnormal tachycardias among the detected episodes, using criteria of greater or lesser stringency depending on the location of the episode in the rate continuum. In response to detection of an arrhythmia, the stimulator will automatically deliver one or more therapies according to the physician's exact prescription (based on various factors including, for example, specific patient data, arrhythmia rate, episode longevity and acceleration or deceleration). The available therapies include bradycardia pacing, anti-tachycardia pacing, cardioverting shocks, and defibrillating shocks, which may be delivered separately or in any combination (according to the physician's prescription) to treat the detected arrhythmia, with an ascending order of aggressiveness of the therapy according to the degree of hemodynamic intolerance of the arrhythmia.
In the invention disclosed in the Haluska et al application, the hierarchy of algorithms developed for detecting arrhythmias in the various rate ranges may be assigned to make the criteria progressively less stringent for detecting episodes in progressively higher rate ranges, so that the detection criteria are relaxed with increasing hemodynamic intolerance of the arrhythmia.
It has been found that in certain circumstances, the detection algorithms may incorrectly detect a particular ventricular response as a reentrant ventricular tachycardia. In such instance, the improper detection would result in the delivery of the selected therapy or sequence of therapies to terminate the VT, when in fact the application of such therapy is unnecessary and undesirable.
Accordingly, it is object of the present invention to provide detection criteria and algorithms which avoid the misclassification of ventricular activity as a reentrant VT.